The main body or door of a refrigerator is provided with a lining material as produced from a sheet of a styrene-based resin, such as an acrylonitrile/butadiene/styrene copolymer (ABS), by vacuum molding or pressure-air molding or a combination thereof (these methods of molding are hereinafter called collectively as "differential pressure molding"). This lining material serves to prevent corrosion of a frame of stainless steel or steel which is subjected to coating with a paint followed by baking, and also provides projections necessary for holding shelves. An attempt has been made to replace ABS with polypropylene which is superior in surface luster and is low in water absorption.
In order to increase the heat-insulating properties of a refrigerator, it has been proposed to provide a urethane foam sheet between the stainless steel plate having a baked coating of the frame and the lining material. In producing such a laminated structure, however, the following problems are encountered.
(1) When a polyolefin-based resin sheet is used as a lining material, adhesion between the polyolefin-based resin sheet and the urethane foam sheet as a heat-insulating material is poor.
(2) Prior to differential pressure molding of the ABS and polyolefin-based resin sheets, it is necessary to remove dust attached onto the surface thereof with cloth, for example. This is more necessary for polypropylene because polypropylene having no polar group attracts dust more easily than ABS.
The problem (1) above can be overcome by employing a procedure comprising the steps of bonding together the lining material and the urethane foam sheet molded into the desired shapes by differential pressure molding, with a liquid acryl or epoxy-based adhesive, thereby producing a laminated member and, thereafter, pressing the laminated member to the inner surface of the stainless steel frame having a baked coating which has been coated with an adhesive. This procedure, however, will increase production cost since it involves a plurality of steps. For this reason, it is preferred to employ an in situ fabrication method as shown in FIG. 1; that is, an expandable urethane solution 4 is injected into a cavity 3 formed between a mold frame 1 and a lining material 2 and then expanded and hardened to produce a composite structure that the mold frame 1, the urethane foam 4', and the lining material 2 are combined together in one body.
The problem (2) can be overcome by introducing an antistatic agent into the sheet in the course of production thereof, or alternatively by coating an antistatic agent solution on both sides of the sheet or at least one side coming into contact with the urethane foam. In accordance with the former method, however, the desired antistatic capability can be obtained only in 30 to 50 days after molding of the sheet. During this period, therefore, it is undesirably necessary to pay attention so that the sheet does not attract dust. For these sheets produced by this method, the satisfactory antistatic capability usually obtained is in 3 to 6 months after molding thereof.
In contrast, the latter method has an advantage in that the sheet can be made antistatic in short periods of time by coating with the antistatic agent solution followed by drying. This antistatic agent, however, is required to have the following properties:
(1) it exhibits good adhesion to the sheet;
(2) it possesses stretch moldability; that is, since the sheet is subjected to differential pressure molding, the antistatic agent is also required to have extensibility as for the sheet;
(3) it is superior in adhesion to urethane foam.